usb: xhci: tegra: mark PM functions as __maybe_unused

usb: xhci: tegra: mark PM functions as __maybe_unused

The added #ifdefs in the PM rework were almost correct, but still
cause warnings in some randconfig builds:

drivers/usb/host/xhci-tegra.c:2147:12: error: 'tegra_xusb_resume' defined but not used [-Werror=unused-function]
 2147 | static int tegra_xusb_resume(struct device *dev)
      |            ^~~~~~~~~~~~~~~~~
drivers/usb/host/xhci-tegra.c:2105:12: error: 'tegra_xusb_suspend' defined but not used [-Werror=unused-function]
 2105 | static int tegra_xusb_suspend(struct device *dev)

Replace the #ifdef checks with simpler __maybe_unused annotations to
reliably shut up these warnings.

Link: https://lore.kernel.org/all/20210421135613.3560777-2-arnd@kernel.org/
Fixes: 971ee247060d ("usb: xhci: tegra: Enable ELPG for runtime/system PM")
Reviewed-by: JC Kuo <jckuo@nvidia.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20211005112057.2700888-1-arnd@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>